1. Field of the Invention
The present invention relates to a power supply unit for use with a portable electronic apparatus and more particularly, a power supply unit and portable electronic apparatus with an AC adapter for converting an AC voltage from an external AC power supply to a DC voltage.
2. Description of Related Art
Currently, there are a variety of personal computers (PC's) available such as desktop, tower and notebook systems. Among them, the notebook PC is designed to be small and light weight in order to provide a portable and transportable use in a mobile environment (i.e., the notebook PC can be used outdoors or at the place it is moved to). A typical example of a notebook PC is an IBM ThinkPad 760 which is sold by IBM Corporation ("ThinkPad" is a trademark of the IBM Corporation).
Almost all notebook PC's are of a battery driven type which can be driven by an internal battery. This is a result of a notebook PC's use in a mobile environment where a commercial electric power source is not available. Conversely, in an office environment where a commercial electric power source is available, an external AC adapter may be attached to a notebook PC to drive the system by a DC input voltage from the AC adapter and charge the internal battery.
It is preferable to use a commercial electric power source which is unlimitedly sustained as a primary power source of a notebook PC rather than an internal battery which has to be cared for its duration or capacity. Consequently, many users that carry a notebook PC also carry an external AC adapter so as to attach it where an AC power source is available. However, while a notebook PC is of a flat shape and a specific form factor (e.g., A4 and B5 sizes) which can be conveniently put into a brief case it is not easy to put an external AC adapter, which is of varying shapes and sizes, into a brief case which is already occupied by the notebook PC. In view of this, a notebook PC of the type which can install an internal AC adapter is recently in advent. An internal AC adapter is an AC adapter which can be contained in the housing of a notebook PC. The internal AC adapter is received in a receptacle space (i.e., media bay) for a detachable storage device such as a CD-ROM drive and a floppy disk drive (FDD).
FIG. 1 schematically shows a powering system of a conventional notebook PC which supports an internal AC adapter.
As shown in FIG. 1, a notebook PC 100 of this type may rely as a primary power source on either a DC output VO of an external AC adapter 10 or a DC output VO' of an internal AC adapter 20. Each output of the AC adapters 10 and 20 is transmitted through output lines 15 and 25 for input in parallel to a DC/DC converter 30. It should be noted that both the external AC adapter 10 and the internal AC adapter 20 are devices for generating a DC output from a commercial electric power source.
The external AC adapter 10 is electrically and detachably connected to the body of the PC 100 by DC inlets 11a and 11b which are provided, for instance, on the wall of the housing of PC 100. The internal AC adapter 20 is exchangeably accommodated, for instance, into the media bay of the PC 100 and is electrically connected to the body of the PC 100 through connectors 21a, 21b. In addition, a power source cable 22 for electrically connecting the internal AC adapter to a commercial power source is detachably plugged in a DC inlet 23 which is exposed on the wall of the housing of PC 100.
The DC/DC converter 30 is a device for converting the DC input to a stabilized DC voltage level which is suitable to drive each hardware element within PC 100 and distribute the output voltage thereof to various portions in PC 100.
The PC 100 is also provided with an internal battery 40 as a primary power source. The battery 40 is usually in the form of a "battery pack" into which a plurality of battery cells are packaged. A re-chargeable battery cell such as Li-ion and NiMH is usually used for the battery cell. The battery 40 is adapted to function as a primary power source of the PC 100 when it is not fed by either one of AC adapters 10 and 20. The AC adapters 10 and 20 are also adapted to charge the battery 40 with their extra electric power. A charge/discharge control circuit 41 controls feeding (charging) the battery 40 as well as feeding (discharging) from the battery 40 to the system.
Incidentally, with an electronic apparatus of the type to which a plurality of AC adapters are attachable, that is, an electronic apparatus having a plurality of AC inputs, a disadvantage exists in that a protecting circuit (e.g., a circuit for protecting from an over voltage or an over current) does not operate properly or the charging current of the battery is excessively large when more than one AC adapter is operated simultaneously. It is therefore necessary to implement in such type of electronic apparatus a mechanism to prevent simultaneous operations of more than one AC adapter or a mechanism for exclusively allowing only one of the AC adapters to supply an output even where a plurality of AC inputs exist.
For example, in FIG. 1, the above disadvantage involved in co-existence of more than one AC adapter can be avoided by providing a switching circuit 50 on the output line 15 of the external AC adapter 10.
The switching circuit 50 is a device for selecting either one of the output VO of the external adapter 10 and the output Vo' of the internal AC adapter 20 and consists of a p-channel MOS FET switch 51 for disconnecting the output line 15 of the external AC adapter 10. The FET switch 51 is serially connected on the output line 15 and has a gate electrode connected to the collector of an npn transistor 53 through a resistor of 20k as an input. The source and the gate electrodes of the FET switch 51 are connected to each other through a resistor of 200k. The npn transistor 53 is provided for connecting/disconnecting the gate electrode of the FET switch 51 to and from ground and has its emitter electrode connected to ground. The emitter and the base electrodes of the transistor 53 are connected to each other through a resistor of 47k. The base electrode of the transistor 53 receives, as an input, the output VO of the external AC adapter 10 (via a resistor of 100k) and the collector electrode of another npn transistor 52 via a resistor of 47k which are ORed together. The transistor 52 is provided for connecting/disconnecting the base electrode of the transistor 53 to and from ground and has its collector electrode connected to the base electrode of the transistor 53 and the emitter connected to ground. The emitter and the base electrodes of the transistor 52 are connected to each other through a resistor of 47k. The base electrode of the transistor 52 receives as an input a detection signal 63 of a detection circuit 60 through a resistor of 47k.
The detection circuit 60 is a device for detecting the DC output VO' of the internal AC adapter 20 and comprises a zener diode 61 connected in parallel to a point P on the output line 25 of the internal AC adapter 20 and a resistor 62 of 20k which connects the other end of the zener diode 61 to ground. The zener diode 61 is an element which has a characteristic in which a reverse current I abruptly increases when a reverse voltage exceeding a zener voltage (Vz: Vz is 5.6 V in FIG. 1) is applied (well known). Therefore, when the output VO' of the internal AC adapter exists, a detection signal 63 is asserted as a result of the reverse current flowing through the zener diode 61. A reverse current blocking diode 70 is inserted in a forward direction in the down stream of the point P on the output line 25 to prevent the detection circuit 60 from being activated by the DC output VO of the external AC adapter 10.
The co-operative operation of the switching circuit 50 and the detection circuit 60 will now be explained hereunder.
It is assumed that the external AC adapter 10 has an AC input while the internal AC adapter 20 has no AC input. In this case, the DC output VO of the external AC adapter is blocked by the diode 70 so that there is no voltage exceeding the zener voltage 5.6 V appearing on the point P on the output line 25. As a result, the output line 63 of the detection circuit 60 is negated and indicates a ground level. The transistor 52 in the switching circuit 50 is switched off due to a low level applied to the base electrode. In response to the transistor 52 being off, the DC output Vo of the external AC adapter in high level is applied to the base electrode of the transistor 53 so that the transistor 53 is switched on. As a result, the FET switch 61 is switched on because its gate electrode is grounded thereby connecting the output line 15 so that the DC output VO of the external AC adapter is supplied to the DC/DC converter 30.
It is next assumed that an AC input is given to the internal AC adapter 20. In this case, the internal AC adapter 20 generates the DC output VO' to apply a voltage exceeding the zener voltage 5.6 V to the point P on the output line 25. As a result, a reverse current I flows through the zener diode 61 to assert the output line 63 of the detection circuit 60. The transistor 52 in the switching circuit 50 is switched on by a high level applied to the base electrode. In response to the transistor 53 being on, the transistor 53 is switched off by ground or low level voltage applied to the base electrode. As a result, FET switch 51 is switched off because the gate electrode is no longer at a low level. With the FET switch 51 being switched off, the output line 15 of the external AC adapter is disconnected and, in place thereof, the DC output VO' of the internal AC adapter 10 is supplied to the DC/DC converter 30.
For example, the IBM ThinkPad 760 series notebook PC (described above) implements a powering system as shown in FIG. 1, which allows an internal AC adapter to be installed. With such a powering system, the AC input is suitably switched depending on the presence or absence of the internal AC adapter so that an erroneous operation of the over voltage/over current protecting circuit of each AC adapter or an excessive powering to the battery is avoided.
It should be noted that a reverse current blocking diode 70 (described above) has to be serially connected to the output line 25 in the feeding system as shown in FIG. 1. It is because, without the diode 70, the output VO of the external AC adapter 10 would flow into the detection circuit 60 so that the detection circuit is unable to exactly detect the presence of the internal AC adapter 20. However, the diode 70 which assumes a significant electric resistance to the current flowing forward is serially connected on the output line 25 for transmitting the main electric power, consuming an electric power of 1.8 W at maximum upon operation of the internal AC adapter 20. Such power consumption is an intolerable waste of electric power and is a heat source within the PC housing. A heat dissipation measure would be required for the heat source, inviting increased consumption of space in the housing as well as an increase in cost.